This invention relates generally to radiation detectors, and particularly to detectors utilized in the nuclear medicine field. Also, this invention relates to a method of detecting high energy radiation.
Radiation detectors capable of sensing high-energy radiation particles such as x-rays, gamma rays, photons, electrons, and neutrons (in general 20 keV and higher), are susceptible to background radiation. If the source radiation or radiation emitted by the object under test varies substantially or is minimal, the background radiation interferes with the sensing capabilities of the detector and ultimately dilutes and distorts the signal obtained from the detector. In that instance, the output signal poorly represents the radiation sought to be detected.
To minimize the effect of this background radiation or non-source emitted radiation, prior art devices utilize lead shielding surrounding the entire detector assembly to exclude the background and other non-source radiation from impinging upon critical parts of the detector.
Detectors utilized in the nuclear medicine field are very susceptible to spurious, background radiation because only a limited amount of a radioactive substance is injected into the person or other living being under test to obtain an anatomical image or to monitor the physiology of an organ. A typical radionuclide utilized for medical diagnosis is technetium-99 m which emits gamma rays at 140 keV. Other radionuclides are iodine 123 which emits gamma rays at 159 keV; Xe-133, 81 keV .gamma.-ray; T.gamma.-201, 70 keV x-ray and 279 keV .gamma.-ray; etc. For example, a small amount of red blood cells (RBCs) are initially labeled with technetium-99 m, (Tc-99 m) and those labeled cells are thereafter injected into the blood stream of a patient (or RBC are labeled with Tc-99 m in vivo) to conduct diagnosis of heart function. A gamma ray camera or a detector assembly (cardiac monitor) is utilized in such a test.
Recently, there is a need for an ambulatory cardiac monitor which is placed onto a patient to monitor the patient's physiological activities during a prescribed period of time. The ambulatory cardiac monitor is placed generally over the heart of the patient and the high-energy detector, mounted within the monitor, senses the ebb and flow of blood through the heart by detection of the gamma rays emitted by the Tc-99 m labeled blood cells. The precise measurement of the gamma rays, emitted by the labeled blood cells, contains a wide range of cardiac information which is helpful in diagnosing, among other things, cardiac disorders. However, the precision of the cardiac monitor is highly dependent upon the ability of the monitor to detect only the gamma rays emitted by the technetium-99 m in the heart to the exclusion of background radiation and spurious radiation present in the other part of body and the ambient environment.
Prior art cardiac monitors, and other types of high-energy radiation detectors, commonly include one or more scintillators or means for detecting radiation which react to the radiation by emitting light of a predetermined wavelength. These scintillation detectors and associated electronic circuits are normally mounted within a lead shield such that radiation emitted from a predetermined source or emanating from a predetermined direction is collimated to impinge upon the scintillators. The light from the scintillators is channeled into some type of photo-detection device which converts the light into an electrical signal. One type of photo-detector commonly utilized is a photomultiplier tube.
The electrical signal output from the photomultiplier tube is applied to a cable which electronically links the monitor to complementary data processing equipment or display means. To insure that only radiation emanating from a certain source or from a certain direction affects the scintillator or scintillation crystals, a lead shield normally extends and surrounds the scintillator, the photomultiplier tube and further includes a lead back shield is utilized to exclude background radiation, traveling in a direction opposite the source emitted radiation, from impinging on scintillator. The use of this extensive lead shielding greatly increases the weight of a cardiac monitor.
In a similar fashion, the weight of other high-energy radiation detectors is greatly affected by the lead shielding normally surrounding substantially all of the detector assembly.